The performance and cost of the pixilated detector arrays are limiting factors for infrared imaging systems. The resolution of an imaging system is governed by the collecting optics and the format of the array. The demand for higher definition images with the same or even wider view angles coupled with size constraints for the overall system limits advances primarily to the imaging array as there is little room for further improvements to the imaging optics. This means that smaller and smaller pixel sizes are required to attain the desires performance improvements. Smaller pixel sizes enable larger numbers of devices of the same size format to be produced on the same substrate thus potentially reducing costs, however the processing challenges associated with smaller pixel sizes dramatically reduce yield contributing to increased cost for the detector arrays. Barrier detectors offer the potential for higher process yields of larger format arrays with smaller pixel sizes that would reduce costs and increase the resolution of infrared imaging systems.
The currently employed strategy for MWIR and LWIR superlattice growth involves epitaxial growth of designed superlattice structures on GaSb substrates with a lattice constant of 6.09 angstroms. The choice of GaSb is due to the commercial availability of GaSb substrates. The only other commercially available substrate near this lattice constant is InAs at 6.05 angstroms. The most common superlattice grown for MWIR and LWIR applications is comprised of InAs and GaSb layers with InSb interfaces to match the lattice constant of the superlattice to the GaSb substrate. However, short minority carrier lifetimes in GaSb containing materials is proving to be a formidable challenge to overcome. More recent studies have incorporated “Ga-free” superlattice absorbers comprised of InAs and InAs(x)Sb(1-x) superlattice layers due to the ability to grow higher quality material with much longer lifetimes. The Ga-free superlattice has the added advantage of being simpler to grow than the Ga containing material. This is due to the reduced number of shutter actions required per superlattice period as a result of removing an element from the recipe, Ga.
Recent results for Ga-free nBn devices grown on GaSb have shown very promising performance in mid-wavelength devices, but low quantum efficiency in long-wavelength devices. Analysis of device data suggests the QE of the long-wavelength nBn devices is being limited by low hole mobility. The ability to grow material with a slightly larger lattice constant would enable the period thickness of the InAs/InAs(x)Sb(1-x) superlattice to be reduced, and increase the absorption coefficient. With this approach a thinner layer of the absorber would be required for operation resulting in shorter diffusion lengths for charge collection thus minimizing the impact of the low carrier mobility.
FIG. 1 shows an exemplary band edge alignment plotted verses lattice constant of relevant III-V semiconductors. Specifically, the vertical dashed lines indicate the currently used GaSb lattice constant and to the right the proposed AlSb lattice constant. It is important to note that the valence band for the Al bearing material increases to the right presenting a more ideal line up between the barrier layer and potential absorber layers. FIG. 1 displays the relative unstrained conduction and valence band energies for the III-V semiconductors commonly used in mid and long wavelength infrared detection. Any semiconductor device architecture that includes a heterojunction will also include band offsets in the conduction band, valence band, or both the conduction and valence bands. If not properly designed, these offsets can have a detrimental impact on device performance by introducing depletion regions and/or forming unwanted barriers to current flow.
FIG. 2 shows exemplary band diagrams demonstrating the importance of valence band lineup for the nBn structure. Part a) shows the flat band diagram for a device structure with a valence band offset (the valence band of the barrier layer is below the valence band of the absorber). Part b) shows this device at operating bias with a significant depletion region in the absorber associated with overcoming the band offset. Part c) shows the band lineup of a MWIR nBn device grown on the AlSb lattice. Note that there is not a valence band offset for this device.
With the nBn architecture a large offset in the conduction band is desired, but an offset in the valence band is detrimental to device performance. As an illustration FIG. 2 a) shows the flat band diagram for an nBn device with a very large valence band offset. In order to operate this device a large external bias will be necessary to overcome the barrier to holes formed in the valence band. FIG. 2 b) shows the band diagram of the same device with a sufficient external bias to overcome the barrier. This device will not only require a high bias to operate, but the g-r component of the dark current will be present at the operating bias due to the depletion region formed in the absorber, thus diminishing the advantage of the barrier architecture. FIG. 2 c) is an example of an ideal heterojunction where the valence band offset places the barrier layer valence band energy slightly above the valence band energy of the absorber layers. To obtain an InAs/InAs(x)Sb(1-x) superlattice device with this lineup lattice matched to GaSb requires the use of a complex quaternary Al(y)Ga(1-y)Sb(x)As(1-x) barrier with sufficient Ga content to raise the valence band energy level of the barrier above that of the absorber. If the device is lattice matched to AlSb, strain balanced InAs/InAs(x)Sb(1-x) superlattice absorbers can be used in conjunction with AlSb binary barrier layer as displayed in FIG. 2c).
Substrates with a lattice constant matching AlSb are not commercially available. However, the use of metamorphic buffer layers affords the possibility to grow infrared detector materials with a wide variety of crystalline lattice constants on commercially available substrates. For III-V based semiconductors, these layers require the precise control of at minimum three constituent atoms, making reproducibility a concern. Any deviation from the designed lattice constant in the buffer layer will carry over into infrared device layers with the potential of diminishing performance. Matching the lattice to a pure binary semiconductor, such as AlSb, is advantageous in ensuring the target lattice constant is achieved. Furthermore shifting the lattice constant to match that of AlSb enables the application of a binary barrier layer, AlSb, opposed to ternary, AlAs(x)Sb(1-x), and quaternary, Al(y)Ga(1-y)Sb(x)As(1-x), barrier layers that are presently used in GaSb lattice matched nBn devices to avoid valence band offsets.